The present invention relates to an offshore structure for disposition in a body of deep water; and more particularly, this invention relates to an offshore structure carrying a platform thereon and held by guy wires secured to anchors on the ocean floor.
Offshore structures in current use in connection with the drilling and production of hydrocarbon deposits in offshore locations have generally included a platform held above the surface of the water by support members which rest on the ocean floor. Such structures, referred to as fixed platforms, have been quite successfully utilized in operations conducted in shallow water. However, as offshore drilling operations move into areas having deeper waters, such as about 300 feet, the fixed platform design becomes less desirable, primarily because of the great expense to fabricate and install such a structure. As a general rule the cost of fabrication and installation of a fixed platform designed in accordance with proven shallow water methods and technology will increase exponentially with water depth.
Although an alternative to the fixed platform design is the floating platform concept, that approach is found to present additional, and perhaps even more serious disadvantages. Floating platforms have the disadvantage of being more susceptible to rough sea conditions and exhibit the undesirable feature of significant heave, pitch and roll motion.
Recently another type of structure for deep water offshore locations have been proposed which has a platform supported on buoyant members that are held in position at the well-site by anchor lines extending to fixed anchors on the ocean floor. Offshore platform structures of this type are referred to generally as "buoyant restrained platform." The basic principle is to provide a platform with buoyant chambers below the wave area to give a positive buoyancy and to tie the structure down to the ocean floor, allowing the buoyancy of the structure to hold the anchoring cables in tension to prevent heaving. Offshore structures of this type offer significant cost-savings for operation in deep-water environments.
Illustrative of the concept of buoyant restrained platform design is that structure disclosed in the article "Tension Leg Offers Steady Base At Sea" in the November 1973 issue of OFFSHORE magazine, beginning at page 100. The structure disclosed there comprises three buoyant vertical columns having horizontal bracing structure. A working deck is positioned on top of the vertical columns above the surface of the water. Several anchoring cables attach to each vertical column and are secured by dead weight anchors on the ocean floor. Anchor loads to hold the structure are significant, however, due to the resistance in the wave zone of the structure.
Additional designs based on the buoyant restrained platform concept are described in the OFFSHORE ENGINEER of May 1975, at page 55. Also, relevant teachings of the buoyant restrained platform concept are found in U.S. Pat. No. 3,256,537 to Clark and U.S. Pat. No. 2,777,669 to Willis.
A yet another approach which has been proposed is that of a structure comprising a slender column held in a vertical position by guy wires extending from near the top of the column to fixed anchors on the ocean floor. The column rests on the ocean floor and extends above the surface of the water with a platform supported thereon. Although the vertical column design has less resistance in its wave zone than the typical buoyant restrained platform design, some resistance is still present and forces are developed from periodic wave motion which act on the structure. A problem associated with guyed structures of this type is that tilting of the column off-vertical makes it very difficult to conduct drilling or production operations. However, increased tension in the guy lines to restrict tilting is not desirable either.
One approach to solving the problem of tilting is that described in U.S. Pat. No. 3,903,705, assigned to Exxon Production Research Company. There is disclosed in that patent a guyed tower platform using a clump weighted guy line system to control swaying and still relieve the structure of excessive moment forces produced by waves. The weights are designed to lift off the bottom during large storm waves, but remain in position during normal sea conditions. The guy line system provides adequate sway restriction, yet, relieves the guy lines of excessive stress and removes the need for heavier anchoring equipment.
Another concept proposed for an offshore structure to provide restricted movement with waves is that of the buoyant tower. The buoyant tower is an elongate tower structure that is held near the ocean floor by a universal joint that permits the tower to tilt. The force required to prevent the tower from tilting excessively is provided by the establishment of buoyancy for the tower near the surface of the water. One design based on the buoyant tower concept is the structure illustrated and described in THE OIL AND GAS JOURNAL of Oct. 28, 1974, beginning at page 60.